Carbon and graphite fibers are currently manufactured by extruding molten carbonaceous materials through fine extrusion holes, and spun into fine threads or filaments. The filaments are subsequently stabilized, i.e. rendered infusible through a heat treatment in an oxidizing atmosphere and then heat treated in an inert atmosphere to convert them into carbon or graphite fibers.
Similarly, thermoplastic fibers are manufactured in mats, roving, and other forms by extruding molten thermoplastic through fine extrusion holes and blowing the extrudate with an air supply. Many problems have been found with respect to shaping and controlling the air supply as well as with controlling the temperature of the molten thermoplastic resin and the air.
Spinning of the carbon or graphite fibers involves using an oxygen rich (air) hot gas to draw the filaments from an extrusion die to produce fibers of very small diameter, as low as about 2 microns. The oxygen penetrates the molten fibers and is entrapped as the fibers cool. The presence of oxygen within the individual fibers assists in stabilizing the fibers in the subsequent steps of the process. Melted fiber precursor pitch is supplied from a suitable tank, fed under pressure through a die by operation of a suitable pump. The molten pitch is expressed through die openings as a series of vertical laterally spaced holes within a melt-blow die into the oxygen rich stream. The compressed air impinges through oblique slots against the extruded pitch material to form a plurality of fine pitch fibers. The die tip is of triangular cross-section, having downwardly, inwardly, and oppositely directed sloping walls fitted into a triangular shaped opening defined by opposed air plates or air lips forming the attenuating air passages. The melted pitch passes through the die openings and upon discharge therefrom, is contacted by the high velocity hot gas streams which pass through the oblique slots angled to intersect just below the die openings. The air streams attenuate the molten pitch fibers and draw them down to a diameter significantly smaller than the diameter of the multiple die openings within the die tip.
Problems have been encountered in maintaining the pitch at proper uniform temperature along the length of the die relative to the hundreds of extrusion holes within the die tip. The utilization of the air streams for fiber attenuating purposes has in some cases materially adversely affected the maintenance of a uniform and set temperature and the extrusion of the pitch under pressure through multiple orifices created by the fine holes within the die head and opening to the apex of the die tip nose. The presence of the air streams have tended to cause build up of the pitch at the tip of the melt blowing die, interfering with the attenuating air stream.
Attempts have been made to improve melt blowing dies to facilitate the fiber or filament drawing process. U.S. Pat. No. 3,825,380 is directed to a die having a special nose configuration of triangular cross-section and particularly suitable for melt blowing of very fine fibers with the design of the melt blowing die eliminating dead spaces on the edge of the junction of two sides of the triangle of the die tip nose where the orifices open at the apex end of the melt blowing die.
U.S. Pat. No. 4,285,655, which is directed to a coat hanger die, employs a formula wherein the radius of the manifold at its inlet is selected in consideration of the flow characteristics of the resin melt to provide a low melt velocity at the inlet for the melt led under pressure to the plurality of extrusion orifices remote from that inlet.
U.S. Pat. No. 4,295,809 provides a mechanism for shifting the air lips relative to the triangular cross-sectional die tip nose for controlling the flow of heated gas blown out through air slots on either side of the die nose. Adjustments are made via appropriate spacers of the set back of the lower face of the air lips relative to the point of intersection of the oblique surfaces of the die tip, as well as the gaps between the air lips and the die tip itself through which the dual air streams pass for intersection downstream of the small diameter holes through which the melt is expressed.
While these patents represent some attempts at improving the operation of the melt blowing die and the creation of uniform melt blown filaments without plugging or stoppage of the melt blowing die producing the same, problems persist within the industry, particularly where the melt material has a relatively high softening temperature.
It is, therefore, a primary object of the present invention to provide an improved melt blowing die, particularly useful in spinning high softening temperature carbonaceous material fibers and their subsequent conversion to carbon or graphite fibers of better uniformity and at lower cost; in which the attenuating air streams have improved controllability; the presence of the attenuating air streams does not adversely affect the creation of and maintenance of the proper temperature of the pitch melt during the extrusion of the same; the air flow streams are thermally isolated from the body of the die; the die has excellent heat stability and control, and wherein the components may be mechanically assembled and disassembled with ease while allowing certain elements to be readily removed without the necessity of dismantling the complete assembly of the melt blowing die itself.
It is desirable to provide an improved process for forming nonwoven thermoplastic web materials of controlled uniformity or nonuniformity as desired by the operator.